Instances may arise when a vehicle begins to sway. Such sway can result in a variety of difficulties which should preferably be avoided. As an example, a trailer may be secured to the rear of a vehicle to be towed, for example, using a ball and receiver type hitch configuration, a pintle hook addition or the like. If the trailer becomes unstable, it oscillates laterally with a frequency and amplitude dependent on the road surface coefficient of friction and the mass of the trailer, including any payload carried by the trailer. A lateral oscillation force is then transferred to the towing vehicle through the tow hitch, wherein the vehicle undergoes sway.
U.S. patent application Ser. No. 11/875,142 (U.S. Application Publication No. 2009/0105906) discloses a method and apparatus for vehicle sway detection and reduction. If a sway requiring reduction is detected, the sway reduction is performed by reducing engine torque and applying independent braking forces to the respective vehicle wheels. The determination as to whether a vehicle is undergoing sway is complicated by the fact that the vehicle's motion could be the result of a combination of sway and slalom (i.e., oscillation due to steering movements of the driver, e.g., to avoid obstacles in the road.) As a result, false detections of sway or slalom may occur. Therefore, it would be desirable to determine if a vehicle is experiencing sway, even in cases where yaw acceleration and lateral acceleration attributable to driver steering movements (i.e., yaw acceleration and lateral acceleration attributable to vehicle slalom) are also present.
According to an embodiment disclosed here, a method for determining whether a vehicle is undergoing sway involves:
sensing yaw acceleration of a moving vehicle with a sensor and providing a yaw acceleration signal;
sensing lateral acceleration of the moving vehicle with a sensor and providing a lateral acceleration signal;
determining whether there is a phase shift of the yaw acceleration signal relative to the lateral acceleration signal;
calculating the magnitude of a phase shift determined in step C; and
comparing the magnitude of the phase shift with a threshold value.
According to another aspect, a vehicle comprises an engine; a plurality of wheels; a braking system configured to apply independent braking forces to each wheel; and a controller operably connected to the engine and the braking system and configured to:
determine whether there is a phase shift of the yaw acceleration signal relative to the lateral acceleration signal;
calculate the magnitude of a phase shift which is determined;
compare the magnitude of the phase shift with a threshold value to determine whether the vehicle is undergoing sway; and
if the vehicle is undergoing sway, perform at least one of: reducing a torque of the engine and applying a braking force independently to each wheel.